1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of speed stages to be established by a selecting operation of a plurality of hydraulic engaging elements. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
As this kind of control apparatus, there is conventionally known one having means for controlling stepwise speed changing for downshifting one stage at a time such that during downshifting control to one speed stage which is one stage lower, upon receipt of a downshifting command to another speed stage which is one stage lower than said one speed stage, a start of downshifting control to said another speed stage is delayed until the downshifting control to said one speed stage is completed.
In this kind of control system for stepwise speed changing, downshifting to a two-stage lower speed stage which is required by the driver of the vehicle, e.g., from the fourth speed stage to the second speed stage, is performed in two downshifting operations of fourth speed.fwdarw.third speed and third speed.fwdarw.second speed. It has therefore an advantage in that speed change shocks are reduced and that the control system can be simplified. On the other hand, since the downshifting is performed one step at a time, the time for speed changing becomes longer than the one in which downshifting is performed directly to the two-stage lower speed stage. Therefore, it sometimes causes the driver to feel that the time lag in the switching of the driving force is large.
As a solution, the applicants of the present invention earlier proposed in Japanese Patent Application No. 253633/1996 the following. Namely, there is provided means for switching a control mode which performs a control to accelerate the time of completion of the downshifting control to one speed stage upon receipt of a downshifting command to another speed stage which is one stage lower than said one speed stage, the downshifting command being issued during the downshifting control to said one speed stage. The downshifting to said one speed stage is thus completed earlier in order to shorten the time of completion of downshifting to said another speed stage.
When the temperature of oil in the transmission is low, the viscosity of the oil becomes high. Therefore, a longer time is required from the time when a control signal has been issued to the time when actual hydraulic pressure in the hydraulic engaging element changes to the hydraulic pressure according to the control signal. As a result, if a control to shorten the time of completion of the downshifting to said one speed stage is performed, the downshifting control to said another speed stage is started before the hydraulic pressure of the hydraulic engaging element to be engaged at the time of downshifting to said one speed stage has sufficiently be increased. The engine will consequently give rise to an excessive racing to thereby cause large shocks.
In view of the above point, the present invention has an object of providing an improvement over the earlier proposed apparatus, in which the occurrence of shocks at the time of low oil temperature can be prevented.